PREVIEW-Cycling-Cancellara the danger man in world road race

FLORENCE, Italy, Sept 28 (Reuters) - Once again, Fabian Cancellara will start a one-day race with a big target on his back in Sunday's elite race at the world cycling championships.

Cancellara's sheer power, combined with his ability on tough, short climbs, make him the clear favourite for the 272.3-km trek from Lucca to Florence.

After 106.6 kilometres on the flat, the peloton will go around a 16.6-km circuit which includes the Fiesole climb (4.4 km with a gradient of 5.2 percent) and the Via Salviati (600 metres at 10.2 percent), where the pure sprinters will be eliminated.

The race will then turn into a duel between the climbers and riders such as Swiss Cancellara and Slovakian prodigy Peter Sagan, 23.

"We will be trying to make it the hardest race possible, to try to isolate the sprinters and make it more of a climber's race," said Tour de France champion Chris Froome, who is not at his best as the end of an exhausting season looms.

"I think Sagan and (defending champion Philippe) Gilbert (of Belgium) are the two to really look out for most. If they get into difficulty with the amount of climbing then I think it would be more of a climber's race but as long as they're there, then it's basically up to them.

"I'm not exactly very punchy or explosive when it comes to a bunch sprint or final kick, so I think if I am to win I'm going to have to try and go clear on possibly the last couple of laps."

Froome and Team GB, which also features Bradley Wiggins, should be helped in that task by other climbers' teams such as Spain and Colombia, who will work for Tour runner-up Nairo Quintana.

RAIN FORECAST

Spain have several cards up their sleeves with Joaquim Rodriguez, Alejandro Valverde, Dani Moreno and Alberto Contador.

Somehow, Froome forgot to mention Cancellara.

The Swiss, who achieved a rare Tour of Flanders/Paris-Roubaix double this year, tried to ease the pressure on himself, saying after claiming the bronze medal in Wednesday's time trial: "I don't want to talk about it.

"Everyone wants to know about Sunday - how I feel, how I go and how I look. There's no way I give that information to you."

The hugely talented Sagan, who won the Grand Prix de Montreal in awe-inspiring fashion this month, will also be a marked man, though less so than Cancellara.

The Swiss showed Sagan how it was done at the Tour of Flanders, when his acceleration in the brief but brutal Paterberg climb left the Slovakian stuck on the road as the youngster settled for second.

Home hopes lie with Vincenzo Nibali, winner of the Giro d'Italia and runner-up at the Vuelta this year, although he has yet to shine in a one-day race.

According to his team mate Alessandro Ballan, Nibali could be the surprise winner even if Cancellara is the man to beat.

"He seemed to be in great shape in the time trial, he has lost some weight, he is fast and powerful," said Ballan, who won the rainbow jersey in 2008 after team leader Paolo Bettini had been marked all day.

"He will be marked and all the riders will be racing against him as they did against Bettini in 2008. That's why we could end up with a surprise winner."

Rain is forecast and could have a bearing on the result.

"In that case, it could be a lottery," said Gilbert, who himself feels "relaxed, having already been world champion".

France, who have had a fine season with four stage wins at the Vuelta, could cause an upset with Warren Barguil, Thibaut Pinot, Thomas Voeckler or Arthur Vichot. (Editing by Clare Fallon)